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SPECT
and Xenon Contrast CT
 Blood
flow to the brain is represented on a color scale, where dark
areas have no flow and bright yellow areas have good blood
flow. The dark "butterfly-shaped" area in
the center of the brain is normal, but the dark area on the
right of each picture is the region of impaired blood supply
(ischemia) that corresponds to the patient's acute stroke.
Prior to t-PA, the SPECT scan showed
a large area of the brain that has lost its blood supply (arrow).
Several
hours after treatment with t-PA, the blood flow has
dramatically improved, although there is still a small area
ischemia (arrow).
SPECT
studies combine nuclear medicine (the use of radioisotopes
in the diagnosis of disease) with computed tomography.
In this technique, the patient either swallows or is
injected with a radioisotope, which travels to a target
organ. Concentrating in the target organ, the
radioisotope emits radiation, which is detected by a gamma
camera that rotates around the patient. The
information obtained via the gamma camera is analyzed by a
computer, which creates a cross-sectional image of the
target organ. SPECT scans are frequently used to
determine if a specific area of the body is receiving
adequate blood flow.
Used
in early hours after infarction, cerebral SPECT is able to
reveal a deficit in local blood flow before changes appear
on CT or MRI [Mohr JP, 1992. Dunbabin DW, Sandercock
PAG, 1991.]. However, SPECT does not reliably
distinguish between hemorrhage and infarction, and it is
unclear whether the method will predict the potential for
clinical recovery. Because it can provide information
regarding cerebral perfusion, SPECT (usually in combination
with transcranial Doppler) has been helpful in following the
course of vasospasm in patients with subarachnoid
hemorrhage.
Stable xenon-enhanced contrast
CT, which uses the inert gas xenon to measure cerebral blood
flow (CBF) in various brain regions, is an alternative to
SPECT. [Dunbabin DW, Sandercock PAG, 1991]. Stable
xenon CT, whereby a patient inhales a mixture of xenon and
oxygen over the period of a few minutes, allows measurement
of their increased in density caused by the gas in brain tissue
[Haubitz B, et al, 1993] and can be incorporated into all
existing CT technologies [Yonas H, et al, 1996]. This
method can be used to determine local cerebral blood flow
in an area as small as 1 x 1 x 5 mm3 area, and
can be repeated within 20 minutes, allowing the assessment
of hemodynamic states [Yonas H, et al, 1996], including (in
certain well-defined settings) the evaluation of acute stroke,
occlusive vascular disease, carotid occlusion testing, vasospasm,
arteriovenous malformations, and head trauma management [Johnson
DW, et al 1991].
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Stroke Education
